In an internal combustion engine, fuel explosion occurs at the top dead center of the piston in a bore, and the explosion pushes the piston downward. Because of such a structure, the temperature of the cylinder bore wall is high on the upper side and low on the lower side. This causes a difference in the amount of thermal deformation between the upper side and the lower side of the cylinder bore wall, so that the upper side significantly expands whereas the lower side less expands.
Consequently, the frictional resistance of the piston against the cylinder bore wall increases, leading to lower fuel efficiency. It is therefore demanded to reduce the difference in the amount of thermal deformation between the upper side and the lower side of the cylinder bore wall.
Attempts have been made to homogenize the wall temperature of the cylinder bore wall by installing a spacer in a groove-like coolant passage and regulating the flow of a coolant in the groove-like coolant passage to control the efficiency of cooling the upper side and cooling the lower side of the cylinder bore wall with the coolant. For example, Patent Literature 1 discloses an internal combustion engine heat medium passage partition member. The partition member is disposed in a groove-like heat medium passage for cooling an internal combustion engine to divide the groove-like heat medium passage into a plurality of passages. The partition member includes: a passage dividing member having a height below the depth of the groove-like heat medium passage and serving as a wall for dividing the groove-like heat medium passage into a bore-side passage and a non-bore-side passage; and a flexible lip extending from the passage dividing member toward the opening of the groove-like heat medium passage and formed of a flexible material in such a shape that its front edge extends beyond the inner surface of one of the groove-like heat medium passages. After the flexible lip is inserted into the groove-like heat medium passage, the front edge comes into contact with the inner surface at an intermediate position in the depth direction of the groove-like heat medium passage, because of its own flexure restoring force, whereby the bore-side passage and the non-bore-side passage are separated from each other.